1. Field of the Invention
The invention relates to multiple access interference suppression in a received signal.
2. Description of the Related Art
In CDMA (Code Division Multiple Access), a narrowband user data signal is modulated using a spreading code having a broader band than the data signal into a relatively broad band. In the WCDMA radio system (Wideband CDMA), the bandwidth is considerably wider, the purpose being the ability to provide a user with increasingly more versatile services in present mobile networks.
Since the spreading codes used by different users are not fully orthogonal, signals transmitted by the different users interfere with each other in the receiver, i.e. multiple access interference (MAI) is generated.
Multiple access interference can be cancelled by the use of an optimal receiver based on MAP (Maximum A Posteriori) or MLSD (Maximum Likelihood Sequence Detection) detection. The problem in these solutions is, however, that the complexity of the receiver increases exponentially with the number of users. Attempts have also been made to cancel multiple access interference by the use of different interference suppression methods in a suboptimal receiver, such methods including MMSE receivers (Minimum Mean Square Error) and interference cancellation (IC) solutions. Interference cancellation solutions include parallel interference cancellation (PIC) and series mode interference cancellation (SIC), a modification of which is groupwise SIC (GSIC). In PIC interference cancellation, the signals of all users are usually detected simultaneously from a received signal, the received signal is regenerated and the detection is repeated utilizing the detected signal and the regenerated signal. This way, interference is suppressed for all users simultaneously, i.e. in parallel. In series mode interference suppression, interference is suppressed successively user-specifically or user group-specifically. Interference suppression may be further enhanced by the use of multistage interference suppression structures. In this case, each interference suppression stage reuses the bit decisions of the previous interference suppression stage to improve performance.
However, suboptimal interference suppression has not eliminated the complexity of the receiver; instead, a plurality of filters are still needed in interference suppression, wherein a large number of multiplications requiring a high computing power are performed using different coefficients.